Method of producing monocrystalline semiconductor rods



W. KELLER Nov. 7, 1967 METHOD OF PRODUCING MONOCRYSTALLINE SEMICONDUCTOR RODS 2 Sheets-Sheet 1 Original Filed NOV. 29, 1963 FIG. 2

W. KELLER Nov. 7, 1967 METHOD OF PRODUCING MONOCRYSTALLINE SEMICONDUCTOR RODS Original Filed Nov. 29, 1963 2 Sheets-Sheet 2 FIG. 4

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United States t t ic 3,351,433 Patented Nov. 7, i967 3,351,433 METHOD OF PRODUCTNG MUNQCRYSTALLTNE SEMiCONDlUCTGR REDS Woifgang Keiier, Pretzfeld, Germany, assignor to Sie= mens Schuekertwerlre Alrtiengesellschaft, Berlin Siemensstadt, Germany, a corporation of Germany Continuation of application Ser. No. scram, Nov. 29, 1963.. This application July 6, 1966, Ser. No. 563,301 Claims priority, application Germany, Dec. 12, 1962, 82,820 8 Claims. (Cl. 23-301) This application is a continuation of Ser. No. 326,969, filed Nov. 29, 1963, now abandoned.

M-onocrystalline semiconductor rods can be produced by pulling them from a melt, according to Czochralski, or by floating zone melting, according to Theuerer. Also known is the so-called pedestal method (Growth and Perfection of Crystals by Doremus, Roberts and Turnbull, published 1958, by Wiley and Sons Inc, New York, treatise by Dash, page 363; also US Patent No. 2,961,- 305) according to which a small mound of semiconductor material is molten, for instance by induction heating, on top of a slotted semiconductor rod, and a monocrystal is then pulled from the melt after immersing a monocrystalline seed therein.

For various reasons, as will appear hereinafter, the known crystal-pulling methods requiring a crucible involve appreciable risk of contaminating the crystal thus making it unsuitable or inferior for electronic or related purposes; whereas the floating-zone and pedestal methods, avoiding such contamination, have been limited to small diameters or also to short lengths of the resulting monocrystals.

It is therefore an object of my invention to produce monocrystalline semiconductor rods of electronical purity that have a larger diameter than heretofore economically producible without running into excessive trouble or impairing the degree of purity. More specifically, it is an object of my invention to produce hyperpure monocrystalline semiconductor rods by pulling them from a melt with the aid of a seed crystal and giving them a diameter larger than mm. or, if desired, a diameter up to mm. and more. It is also an object to permit giving such monocrystals a greater length and hence a larger volume than obtained with the known methods.

To achieve these objects, and in accordance with a feature of the invention, I mount a massive and relatively thick semiconductor rod in vertical position so as to leave the top face unattached, the diameter of the thick rod being larger than, for example more than twice or three times, the diameter of the monocrystalline rod to be produced. Thereafter, I apply heat to the top face and concentrate the heating in a center area of the top face so that the annular margin area of the top face is heated to a lower temperature than the center, thus producing on the top face a melt laterally held in position by the top-face margin. Now, I immerse a crystal seed from above into the melt and, while continuing the heating as described, withdraw the seed in the vertically upward direction at such a speed that the growing monocrystal assumes the rated diameter.

This method has the advantage that, on one hand, monocrystals with large diameters can be produced and, on the other hand, the absence of a crucible precludes the danger of contamination. Up to now the production of monocrystals more than 25 mm. in diameter by floating zone melting has been ditlicult and monocrystal diameters in excess of 35 mm. have been virtually beyond feasibility. The above-mentioned pedestal method encounters similar limitations and affords producing an only short monocrystal rod in each individual pulling operation, be-

cause of the small quantity of molten material which is not replenished from the slotted and not electrically heatable pedestal. Although monocrystals of larger diameters can be produced by pulling them from the crucible which permits regulating the diameter of the growing monocrystal by proper temperature control of the melt and/ or the pulling velocity, this process has the disadvantage that impurities, particularly oxygen, are diffused into the melt from the wall of the crucible. Further difficulties are encountered in the case of high-melting material, such as silicon, since the wall of the crucible then commences to become plastic. Therefore, the Czochralski method (pulling from the crucible) is actually employed only for germanium and some A B -compounds,

The foregoing and more specific objects and features of the invention will be apparent from the following description in conjunction with the accompanying drawings which, by way of example, show various embodiments of devices suitable for practicing the process according to the invention.

FIG. 1 shows a vacuum vessel in which the process can be carried out.

FIG. 2 shows a portion of FIG. 1 on enlarged scale.

FIGS. 3, 4 and 5 show other types of embodiments relating to the process according to the invention.

The process according to the invention can be carried out by means of protective gas (in a non-oxidizing atmosphere), as well as in a vacuum. FIG. 1 shows a vacuum vessel designed for this purpose. A box-type housing 2 is equipped with a glass window 3 through which the actual process within the vessel can be observed. The vessel can be evacuated by means of a connecting duct 4. Located within the vessel is a thick rod 5 in coaxial relation to a thin rod 6 produced from the thick rod. Between these two rods, there is a melt 7 produced and kept heated by concentratable heating means such as electron rays or an induction coil. In the illustrated embodiment an induction coil 8 is used. It is mounted on an elongated tubular carrier 9 which extends to the outside through an airtight seal it) in the bottom of the vessel 2. The carrier 9 contains the insulated electric leads for the heating coil, as Well as the inlet and outlet tubes for a coolant which is designed to cool the heating coil. In this respect the carrier 9 and coil 8 may be in accordance with US. Patent 2,904,663. The arrow it serves to indicate that the carrier 9 with the heating coil 8 can be displaced vertically from the outside.

A lower mounting 12, attached to a guide bar 13 holds the thick rod 5. The guide bar 13 also passes to the outside through an airtight seal 14 and can be moved in a vertical direction from the outside. The thinner rod 6 is held in a similar fashion in an upper mounting 15 which is attached to a shaft 16. This shaft 16 also passes to the outside through an airtight seal 17 and can be moved from the outside in a vertical direction, as well as rotated about its own axle.

It is essential to the invention that the melt 7 is heated in such a manner that it can be held in place by the rim of the top face of the thick rod 5 (FIG. 2). This is achieved primarily by bunching the heating effect. One way of doing this is to concentrate electron rays or heat (infrared) rays approximately in the center of the top face of the thick rod 5. Another, or an additional way is to construct an induction-heating coil in such a manner that the desired bunching of the heating effect is achieved. The illustrated embodiment shows such a heating coil 8 in the form of a pancake helical winding. However, the heating coil may also be designed in the form of a cylindrical winding. FIG. 2 shows in addition a reflector 18 which reflects the transmitted heat in order to preheat the thick rod 5 and reheat the thin rod 6, For instance, silver foil, bent cylindrically and enclsing the melt, can thus be used as a reflector. A vertical slot in the foil prevents electric coupling with the heating coil. The pulling process can be started up as follows. First the thick rod 5 is mounted on its holder 12, leaving the top face of the rod exposed, and the seed 6 is mounted on holder but kept axially away from the top of rod 5. Next the top of rod 5 is heated by infrared radiation focused onto the center area of the top face, the heat source being preferably mounted in the vessel, although it may also act from the outside through a lateral window similar to that shown at 3. Also applicable for initial heating is an electron gun mounted in the vessel and directed onto the center area on the top face of rod 5. During initial heating the coil 8 may be kept away from the top face. However, when the top face commences to become red hot, the coil 8 is placed close to the top-face plane to raise the temperature to the melting point. Thereafter only induction heating by coil 8 is needed for maintaining the melt on top of rod 5. After lowering the seed until its tip dips into the melt, the crystal pulling proper can commence and is preferably conducted in the following manner.

Preferably, the heating coil should be supplied with hi h frequency in the short-wave range, for instance with approximately 5 megacycles per second. The heating power amounts to approximately 5 to 10 kw. The growing monocrystal is being rotated around its own axle in the usual manner, so that an axially symmetrical growth is assured. The preferred speed of rotation is between 10 and 150 r.p.m., and may amount to 40 rpm. for instance.

The heating coil 8 may be kept stationary relative to the vacuum vessel, merely pushing up the thick rod 5 from below, while the thin rod 6 is being pulled out upwardly. The upward pulling speed of the rod is preferably kept at 2 to 4 mm./sec. in relation to the heating coil. If desired, the thick rod 5 may be kept stationary, and the heating coil 3 is then moved downward with the aid of the carrier 9. The ratio between the diameter of the thick rod 5 and the diameter of the thin rod 6 should preferably be greater than 1.4. For example, the thick rod 5 may have a diameter of 60 mm. for a thin-rod diameter of 35 mm.

At the beginning of the process it is advisable to immerse a crystal seed with a considerably smaller diameter, for instance 6 mm., into the melt 7 produced in the top face of the thick rod 5, whereupon the pulling speed may be regulated in order to produce the desired diameter of the growing rod 6. Experience has shown that the quality of the crystal can be improved by selecting a crystal seed with a smaller diameter.

Applicable as a thick rod 5 is a polycrystalline semiconductor rod obtained by pyrolytic dissociation and precipitation from the gaseous phase, for example in accordance with the process described in Patent 3,011,877.

FIG. 3 shows another example of the process according to the invention, in which the rim of the top face of the thick rod 5 does not only remain intact and thus is capable of holding the melt 7, but in which the melt rests on the bottom of a crucible formed by the semiconductor rod itself. In other words, the rim of the thick rod remains intact even after this rod, for the purpose of replenishing the melt, has been moved up. Thus the rim forms the wall of a crucible as the melt advances toward the foot of the rod. Accordingly, FIG. 3 shows the melt 21 kept molten by the heating coil 22. while located within a cavity in the thick rod lit. The thin monocrystal rod 23 is being pulled from the melt in the upward direction.

The walls of the crucible cavity produced in the described manner may be of such a nature that the melt may be laterally supplied with additional semiconductor material. For instance, FIG. 3 shows a slot 24 through which powdery or granular semiconductor material is added to the melt. As illustrated in FIG. 3, there is provided an additional induction heating coil 25 into which a semiconductor rod 26 is being introduced from the top in a diagonal or vertical direction. The lower part of the semiconductor rod 26 is inductively heated to such an extent that the material melts and drips off, thus replenishing the melt 21 with liquid semiconductor material. This affords supplying the melt 21 with a continuous flow of semiconductor material at the same rate at which the material is being removed from the melt by the growing rod 23. Therefor, all parts of the entire apparatus, with the exception of the rod produced 23, may be stationary.

FIG. 4 exemplifies another way of practicing the invention. A thin rod 32 is being pulled in an upward direction form a melt 31 located in a thick rod 3%. The melt 31 is inductively heated by a heating coil 33. Through the heating process, the melt acquires such a form that it advances into the thick rod. An additional heating coil 34 serves to melt the semiconductor material that would otherwise remain solid along the rim. The melting material then flows, in the form of droplets into the melt 31 at the edge of the depression created by the melt 31. The semiconductor rod 30 within the area of the heating coil 34 is heated in such a way that the edges which are to be molten continuously melt towards the center. This can also be achieved if the melt 31 supplies continuous radiant heat to the interior wall of the depression thus formed. Furthermore, a portion of the heating effect of the induction coil 33 also acts upon the interior edge of the retained edges of the cavity.

In the embodiment shown in FIG. 5, a thin rod 41 is derived from a thick rod whose top face carries a melt 42 produced by an induction coil 43. According to experiments, the crosssectional shape of the melt is approximately of the wavy configuration illustrated in FIG. 5, although, for emphasis, the shape of the melt has been shown somewhat exaggerated. In the center of the melt, there remains an elevation of still solid material, the induction heating being primarily effective in the immediate vicinity of the turns of the heating coil 43. This effect can be reduced by pre-heating the thick rod with the aid of a reflector as shown in FIG. 2, which reflects radiant heat back to the semiconductor material. A better remedy is to provide for additional heating, for example with the aid of an additional induction coil 4d, illustrated in FIG 5. If the thick rod 44] has a diameter of approximately 60 mm., the pre-heating coil 44 may surround the thick rod along 40 mm., starting 20 mm. below the top face down to approximately 60 mm. It is advantageous to adjust the heating effect of the pre-heating coil 44 in such a manner that the temperature of the semiconductor material in that area approachces 1200 C. if silicon is used. It will be understood, however, that the invention is also applicable to germanium, III-V semiconductors and any other semiconductor substances amenable to crystal pulling operations.

However, instead ofor in addition to-the methods described above, heating of the entire semiconductor material may also be carried out by direct flow of current. This can be done, for instance, by designing the seals 14 and 17, shown in FIG. 1, as electric insulations, and by supplying electric current through the parts 12, 13, 15 and 16. Direct or alternating current of or c.p.s. at a current density of A./cm. may be used for this purpose.

I claim:

1. In a method of producing a monocrystalline semiconductor rod of more than 25 mm. diameter by pulling it from a melt with the aid of a crystal seed and a pair of induction heating coils, the steps of forming a downwardly extending recess from a solid rim in the upper end of a vertically disposed thick semiconductor rod which supports molten material created therein, inserting a first one of the said induction heating coils, having dimensions smaller than the horizontal cross section of that of the recess, into the recess for heating semiconductor material therein to the molten state, and placing the second induction heating coil at a location above the first coil and extending over the solid rim of the thick rod surrounding the recess so as to concentrate the heating action thereof on at least a portion of the rim to cause solid material located at said rim to become molten and drop into said recess.

2. Method according to claim 1, which includes placing a supply rod for replenishing semiconductor material pulled from the melt so that an end thereof is located in the magnetic field of the second induction heating coil.

3. Method according to claim 1, which includes disposing an additional induction heating coil around the thick semiconductor rod and energizing the additional heating coil to heat the thick rod to a temperature below the melting point of the semiconductor material thereof.

4. Method according to claim 1, which includes energizing the second induction heating coil so as to produce a magnetic field of such strength as to melt the solid rim of the thick rod surrounding the recess formed therein.

5. Method according to claim 1, which includes adjustably displacing the thick semiconductor rod and the first induction coil relative to one another in the direction of the longitudinal axis of the thick rod.

6. Method according to claim 5, which includes maintaining the first coil and the melt stationary during crystal pulling while advancing the thick rod upwardly to progressively replenish the melt by melting material of the thick rod.

7. Method according to claim 5, which includes maintaining the thick rod stationary while pulling the monocrystal upwardly from the'melt, and progressively shifting the first induction heating coil downwardly for replenishing the melt by melting material of the thick rod.

8. Method according to claim 5, which includes pulling the seed from the melt so as to produce the melt containing recess in the top face of the thick semiconductor rod, and continuing the pulling operation so that the recess advances axially into the thick rod for replenishing the melt by melting material of the thick rod.

References Cited UNITED STATES PATENTS 2,743,199 4/1956 Hull 23-301 2,992,311 7/1961 Keller 23301 2,999,737 9/ 1961 Siebertz 23-273 X 3,036,892 5/ 1962 Siebertz 23-273 X 3,113,841 12/1963 Reuschel 23-273 FOREIGN PATENTS 1,235,341 5/1960 France.

775,817 5/ 1957 Great Britain.

908,370 10/ 1962 Great Britain.

911,360 11/ 1962 Great Britain.

347,579 8/ 1960 Sweden.

NORMAN YUDOFF, Primary Examiner. G. P. HINES, Assistant Examiner. 

1. IN A METHOD OF PRODUCING A MONOCRYSTALLINE SEMICONDUCTOR ROD OF MORE THAN 25 MM. DIAMETER BY PULLING IT FROM A MELT WITH THE AID OF A CRYSTAL SEED AND A PAIR OF INDUCTION HEATING COILS, THE STEPS OF FORMING A DOWNWARDLY EXTENDING RECESS FROM A SOLID RIM IN THE UPPER END OF A VERTICALLY DISPOSED THICK SEMICONDUCTOR ROD WHICH SUPPORTS MOLTEN MATERIAL CREATED THEREIN, INSERTING A FIRST ONE OF THE SAID INDUCTION HEATING COILS, HAVING DIMENSIONS SMALLER THAN THE HORIZONTAL CROSS SECTION OF THAT OF THE RECESS, INTO THE RECESS FOR HEATING SEMICONDUCTOR MATERIAL THEREIN TO THE MOLTEN STATE, AND PLACING THE SECOND INDUCTION HEATING COIL AT A LOCATION ABOVE THE FIRST COIL AND EXTENDING OVER THE SOLID RIM OF THE THICK ROD SURROUNDING THE RECESS SO AS TO CONCENTRATE THE HEATING ACTION THEREOF ON AT LEAST A PORTION OF THE RIM TO CAUSE SOLID MATERIAL LOCATED AT SAID RIM TO BECOME MOLTEN AND DROP INTO SAID RECESS. 